The invention relates to a device and method for determining the need for regeneration in a NOx storage catalyst arranged in the exhaust the channel of an at least-temporarily lean-running internal combustion engine for a motor vehicle.
Modern combustion engines are preferably operated under lean conditions to optimize fuel consumption. Under these conditions, a specific fuel consumption has typically a minimum. This minimum occurs, for example, for gasoline engines at an air-fuel ratio of approximately λ=1.1 (gasoline engines with direct fuel-injection can have lambda values approaching λ=4.5 and Diesel engines can have lambda values approaching λ=8). Under certain driving conditions, operating the motor vehicle requires maximum torque from the internal combustion engine (for example, kick-down, acceleration). The internal combustion engine supplies a maximum torque at an air-fuel ratio of λ=0.95 to 0.98, which disadvantageously lies outside the operating range where the fuel consumption is optimized.
The pollutants emitted by the internal combustion engine are mainly dependent on the air-fuel ratio in the combustion process. If an air-fuel mixture is burned under rich conditions (λ<1), then the quantity of naturally reducing pollutants, such as carbon monoxide CO and unburned hydrocarbons HC, increases significantly. On the other hand, the formation of nitrous oxides NOx is enhanced when changing to a lean atmosphere. The formation of NOx goes through a maximum, which lies approximately in the range where the fuel consumption of the internal combustion engine is optimized. Accordingly, if the internal combustion engine runs at least temporarily under lean conditions to reduce fuel consumption, then a high NOx emission level has to be tolerated.
It is also known to clean the exhaust gas produced in the combustion process and pass the exhaust gas through catalysts arranged in the exhaust channel of the internal combustion engine. Under rich conditions, the reducing agents HC, CO can then at least temporarily almost completely react with the remaining residual or stored oxygen on the so-called oxidation catalysts. The emission of pollutants can be reduced by a substantial amount under stoichiometric conditions through the use of so-called three-way catalysts, which have an additional catalyst component supporting the reduction of NOx. NOx is then reacted with the reducing agents HC, CO.
The required mass flows of the reduction agents (HC and CO mass flows) are too small under lean conditions, i.e., in particular in the range around λ=1.1 where fuel consumption is optimized, to enable a complete reduction of the NOx emission. It is the therefore known to associate with the reduction catalyst a NOx absorber (NOx storage catalyst), which stores the NOx until a sufficient quantity of reducing agents can again be provided. Such NOx storage catalyst has, of course, a finite storage capacity and needs to be regenerated in regular intervals by exposing the NOx storage catalyst to a rich or stoichiometric exhaust gas. Such process should be controlled so as to, on one hand, minimize a NOx breakthrough emission caused by an depleted storage capacity and, on the other hand, ensure that the operation under rich or stoichiometric conditions with unfavorable fuel consumption is not forced more often than necessary.
It is therefore known to control the regeneration by defining a need for regeneration based, for example, on the NOx breakthrough emission measured downstream of the NOx storage catalyst. If this emission exceeds, for example, a predetermined threshold value, then the aforedescribed actions are initiated. However, such an inflexible definition of the need for regeneration can unnecessarily increase the fuel consumption under certain operating conditions. For example, acceleration processes can also be performed under lean conditions where fuel consumption is optimized. It therefore makes little sense to perform a regeneration only in a subsequent driving phase at an approximately constant speed or with a small load, because the relatively low mass flow of the reducing agents can then prolong the regeneration. Moreover, the difference in fuel consumption between lean operating conditions and corresponding stoichiometric or rich operation is greater with smaller loads than with heavier loads.